The present invention relates generally to exposure, and more particularly to methods for forming a pattern on a mask or reticle (these terms are used interchangeably in this application). The present invention is suitably applicable, for example, to exposure apparatuses and methods, device fabricating methods, and devices fabricated from an exposed object or a target object, wherein the exposure apparatus and method are used to fabricate various devices including semiconductor chips such as ICs and LSIs, display devices such as liquid crystal panels, sensing devices such as magnetic heads, and image pick-up devices such as CCDs, as well as minute contact hole patterns used for micromechanics. Here, the micromechanics is technology for applying the semiconductor IC fabricating technique for fine structure fabrications, thereby creating an enhanced mechanical system that may operate at a level of micron.
The conventional photolithography has used a projection exposure apparatus that transfers a mask pattern onto a wafer through a projection optical system. Various improvements have been proposed for the fine photolithography process, which generally include a shorter exposure wavelength and/or a larger numerical aperture (NA) of the projection optical system in the projection exposure apparatus.
Mask patterns include an adjacent and cyclic line and space (L & S) pattern, and a contact-hole pattern that has a line of adjacent and cyclic contact holes (i.e., arranged at the same interval or pitch as the hole diameter). Generally speaking, the L & S pattern is more easily resolved than the contact-hole pattern. Therefore, there has been a demand to expose a line of contact holes with resolution as high as that of the L & S pattern.
As a solution for this problem, a method for forming fine contact holes has recently been proposed which arranges, around a desired contact-hole pattern, a dummy or auxiliary contact-hole pattern having a hole diameter smaller than that of the desired contact-hole pattern (these terms, i.e., “dummy” and “auxiliary”, are used interchangeably in this application) so as to resolve only the desired contact-hole pattern.
As this method deals with the desired pattern that includes only a line of periodic contact holes, the way of arranging the auxiliary pattern is clear. However, the way of arranging the auxiliary pattern has not conventionally been proposed for such a desired pattern as blends a line of contact holes and isolated contact hole(s). In particular, the recent semiconductor industry has been shifting its production to a system chip that includes highly value-added and various types of patterns, and thus it has become necessary to form plural types of contact-hole patterns on a mask.
Conventionally, designers have manually arranged an auxiliary pattern with trial and error for a desired pattern that blends a line of contact holes and isolated contact hole(s), and this case has a problem in that the desired pattern cannot necessarily be exposed with high resolution. The auxiliary pattern is originally added to enhance the periodicity of the desired pattern, and the periodicity of the desired pattern depends upon a combination of contact holes. It is not always easy to combine which contact holes in which directions.
On the other hand, it is conceivable to use double exposure (or multi-exposure) that employs two masks and expose different types of patterns independently. However, the double exposure has many practical issues to be solved including the increased cost due to two masks, lowered throughput due to two exposures, and high overlay accuracy over two mask exchanges.